Osteoporosis, a significant health problem in US, is a disease with imbalance between the processes of bone formation and bone resorption. It is known that mechanical loading can influence these processes. Our long-term goal is to elucidate the molecular mechanisms of bone cell response to mechanical loading. Over the past several years, substantial evidence has indicated that extracellular nucleotides, such as ATP, signaling through P2 purinergic receptors, play an important role in the regulation of bone behavior. We and others demonstrated that mechanical loading induced fluid flow causes ATP release in bone cells, and ATP subsequently activates calcium signaling pathways via P2Y purinergic receptors. P2Y receptor is a G protein-coupled receptor (GPCR), which is universally critical in normal biological processes. However, the role of P2Y receptors in bone biology, particularly in bone mechanotransduction, is unknown. Our preliminary data suggest that P2Y2, one subtype of P2Y receptors, activated by ATP is involved in changes in gene expression in response to fluid flow. In addition, desensitization of a GPCR's has recently been shown to be an important component of the mechanosensing apparatus in bone. Our results suggest that G-protein coupled receptor kinase 2 (GRK2) is involved in the desensitization of P2Y activation in response to fluid flow. Interestingly, our preliminary results suggest that P2Y2 deficient mice exhibit moderate bone phenotype. More importantly, our data suggest that the osteogenic response to mechanical loading in P2Y2 deficient mice is suppressed. Thus, our central hypothesis is that biophysical signals, such as fluid flow, regulate bone cell behavior via a mechanism involving P2Y purinergic receptors which are desensitized by G-protein coupled receptor kinases. To test this hypothesis we will conduct a series of in vitro and in vivo experiments to examine the role of P2Y receptors in bone cell mechanotransduction (aim 1), the regulation of P2Y receptors by GRK (aim 2) and parathyroid hormone (PTH) (aim 3), and mechanical loading effects on intact bone from mice deficient in P2Y and GRK (aim 4). PUBLIC HEALTH RELVANCE. Osteoporosis is a significant health problem that affects over 44 million Americans. The proposed project is to determine the molecular mechanism responsible for mechanotransduction in bone via P2Y receptors. Completion of this project will ultimately lead to novel targets for pharmacological intervention in bone diseases that have a mechanical component, such as osteoporosis.